The theme of this proposal is that as humans and animals age, internal control of many rhythmic and homeostatic functions weakens. This is seen in more feeble circadian rhythms, less synchrony among those rhythms, and inability to maintain body temperature during cold or heat challenge. One consequence of this deterioration may be that external variables like ambient temperature and the light/dark cycle become more important in supporting individual homeostatic systems and their circadian rhythms, and also in synchronizing rhythms. Desynchronization of normal phase relationships may be a concomitant of aging and a significant contributor to decreased function of physiological systems. This research program will use a circadian rhythm framework to assess the deterioration of several circadian rhythms with age in individual rats, with particular emphasis on the circadian rhythm of body temperature, to determine whether relieving thermoregulatory problems (e.g., through manipulation of environmental temperature) could alleviate problems that are a common accompaniment of aging. For instance, elderly humans and rodents do not respond well to thermal challenges. Old people are at risk for accidental hypothermia, a condition in which body temperature falls even at normal room temperatures. This may be a result of internal temporal desynchronization between heat production and heat loss mechanisms, and we can model this in old rodents. Fever is often difficult to identify in old people, and we are proposing several experiments to study this in aging rodents. The proposed work may also contribute to a drug-free way to improve sleep in aged populations. Sleep, especially REMS, is extremely sensitive to the thermal environment, falling off as the ambient temperature deviates from the zone of thermal comfort. Anything that affects temperature regulation may make a particular ambient temperature more or less stressful, and thereby alter sleep. This means that an ambient temperature that was comfortable when the animal was younger may be stressful as it ages, and therefore sleep will be disrupted. The work outlined in the present proposal asks important questions, the answers to which could lead to practical changes in the way many elderly people lead their lives and make them more functional. Starting with sorely needed descriptions of phenomena and identification of linkages, and then evaluating the effects of environmental manipulation on thermoregulation and sleep, and measuring where the deficits in fever production lie in elderly rats, could have immediate application to similar problems in humans.